In recent times, the electronics industry has moved towards more compact integrated circuit packages with surface-mounted terminals. The density of wiring patterns have tremendously increased within the recent past. Therefore, many circuit board applications today utilize discrete wiring technology which includes insulated wires crossing over each other. Consequently, signal paths can run in X, Y, and non-axial directions on the same substrate.
Accordingly, insulated wire is scribed into an insulating substrate by feeding a continuous strand of wire onto the surface of the substrate while simultaneously affixing the wire and cutting it at predetermined interconnection points. Thus, a wire image of a predetermined interconnect pattern results, which includes discrete or discontinuous wire pieces affixed to the substrate. A wire scribed circuit board is described in U.S. Pat. No. 3,674,914.
Interconnection points exist at the ends of wires and at certain intermediate points along wires in the form of metal plated holes which provide conductive paths to the terminals of external components.
The locations and the sizes of the holes are predetermined and replicable for each wire scribe board. Generally, it is preferred to first apply the insulated, preformed conductor wires to the board surface along a pre-programmed path and to then drill or lase the board at hole sites which are those locations at which terminals are to be located. Plating material may then enter the holes, connecting the wire terminations to external pads.
Once a discrete wiring board has been scribed, it is necessary to inspect the board for any wiring errors. Conventionally, a human operator may inspect the board visually. However, for a typical board there is a multitude of wires, which renders visual inspection highly inefficient and error-prone. The problem is critical at interconnection sites. Since the scribed wires at drilling points are stripped for later connections, any scribing error may cause a "short" or an "open" at an interconnection site.
Automatic optical inspection systems have been quite useful in detecting faults compared to visual inspection by human operators. Conventionally, automatic optical inspection machines have been implemented based on two fundamental methods. The first method is the design rule approach. This method scans a board in search of minimum design rule violations which are detectable within the span of a very small window. Minimum trace width and trace separation violations, as well as minuscule defects like scratches are targets of the design rule check.
The second method is the reference comparison approach. The reference method compares the test board, pixel by pixel to a representation of a "golden board" stored in a memory.
A prevalent automatic optical inspection architecture includes panel illumination with incoherent light, and image detection with a monochrome charge coupled device (CCD) array. For such systems, inspection of artwork and hole sites is provided by back-lighting, and inspection of conductor against substrate is accomplished by top-lighting. In back-lighting, the illumination and detection hardware are located on opposite sides of the article under inspection. In top-lighting, the detection hardware and illumination emitter are located on the same side.
Automatic optical inspection with top-lighting is based on illuminating the board and detecting different intensities of reflected light. Thus, conductor and substrate may be recognized by the amount of their reflected light. For example, bright shiny copper can be easily differentiated from dull dark substrate.
However, inspection of copper oxide is not as easy. A difficult inspection task is detection of faults at solder sites. Difficulties arise because solder has an irregularly shaped three dimensional surface and is very shiny. These qualities cause it to reflect light in an irregular manner. Inspection of discrete wiring boards has proven to be very difficult as well. Like solder, wire has a curved three dimensional surface which reflects light irregularly. The way the wire was scribed, whether adhesive has oozed over it, whether it has suffered any abrasion and the existence of nearby crossovers, are all determinants as to the manner in which it reflects light. Furthermore, a discrete wiring panel prior to encapsulation is strewn with topographic features other than wire. Bumps, scratches, troughs and ridges in the soft shiny substrate can result in bright reflections which act as wire impostors to conventional automatic optical inspection systems.
For example, when viewing a discrete wiring board illuminated from the top, the distribution of wire and substrate brightness fall into two broad overlapping domains. In the region of overlap, analysis based solely on light intensity is subject to potential confusion as to the type of material present. Consideration of spatial structure can help to reduce ambiguity, but it is computationally expensive, and often still not decisive. Thus, it becomes necessary to treat marginal decisions conservatively, to err in favor of rejecting good points on a panel rather than authorizing bad points. Such rejection criteria are a further disadvantage with conventional automatic inspection systems.
The task of differentiating wire from substrate becomes even more difficult when the color of the scribed wire is similar to the color of the substrate. In this event, the wire and substrate brightness can overlap substantially, causing many false rejections.
Thus, there is a need to inspect highly dense circuit boards with high efficiency and reliability, without the disadvantages described hereinabove.